Discovery of Ordovician-Devonian strata in the south of the Qiangtang area, Tibet

Low-grade metamorphic rocks are exposed extensively in the Qiangtang area, Tibet. There is a great dispute about the age of the strata because of no reliable fossil evidence and other chronological evidence. Cephalopods, crinoids, graptolites and tentaculites fossils are discovered at Tashi Hill north of Gangtang Co. Primary identification indicates that the cephalopod is Sinoceras,Michelinoceras, Wennanoceras, Archigeisonoceras etc. and the crinoid is Monograptus. The Sinoceras-Michelinoceras assemblage is the key fossil assemblage of Mid Ordovician Baotaan age, which reflects the Mid Ordovician aspects. There are several tens of meters of fossil-free clastic rocks below the Mid Ordovician cephalopod-bearing horizon, which are probably Ordovician sediments. The discovery of Ordovician-Devonian biotas and strata provide new important evidence for the stratigraphic division and correlation and the study of the tectonic evolution of the Qiangtang area.

Discovery of early paleozoic strata in south of Qiangtang, Northern Tibet and its significance

Due to the lack of fossil evidence, the age of Amugang Group, previously referring to the large area of metamorphic formations developed along the south of Longmucuo-Shuanghu suture in southern Qiangtang area, northern Tibet, has been under heated debate. Some geologists were convinced that the age of metamorphic formations was of Pre-Devonian according to the emergence of nonmetamorphic fossil-beating Devonian in Chasang area, north of the suture. Most geologists believed that its age was of Pre-Sinian. From 2003 to 2004, we collected a large amount of Nautiloid and Graptolite fossils in upper metamorphic formations of Mayigangri area and determined the age to be middle and Late Ordovician-Silurian. It is the first time to discover the Early Paleozoic strata based on reliable fossils, which not only provides evidence to its age ascription, classification of the strata and its correlation to adjacent areas but also offers data to lithofacies, palaeogeography and tectonic pattern of studied area.

Discovery of Early Devonian strata in northern Qiangtang, Tibet, and the establishment of the Pingshagou Formation

Although it is important to determine whether there exist sedimentary strata older than the Upper Permian in the northern Qiantang area in Tibet, there has been no report of such old strata in this area. During the geological mapping of the Mayigangri area, we discovered strata that contain the bivalves Eoschizodus roemeri (Beushausen), E. minor (Beushausen) , E. infiatus (Roemer), Actinodesma (Actinodesma) cf . maneiforme Sandberger, A . (Actindesma) cf . vespertilio Maurer, and the brachiopod Huananochonetes subquadratus Sun & Chen. These fossils indicate a late Early Devonian age (Emsian) , thus the strata represent the first discovered Lower Devonian rocks in northen Qiangtang, confirming the existence of strata older than the Upper Permian. A new stratigraphic term, the Pingshagou Formation , is introduced. The new data provide constraints on the tectonic, palaeogeographic, and palaeobiogeographic history of the north Qiangtang area in the Early Devonian.

Discovery of the Early Ordovician strata in Xianza County, Tibet and its significance

There is no any sediments contain fossils which formed earlier than Middle Ordovician in northern Tibet yet. It is very important whether have developed the Cambrian to Early Ordovician sedimentary strata. During geological mapping of Xainza County in 2000, Tetragraptus approximatus, the typical one of graptolite belts which cosmepolitically distributed in the Arenigian of Early Ordovician, was collected from the so-call Pre-Sinian Nianqingtanggula Group. So there is developped the Early Ordovician sediments and we have established Zakang Formation and Taduo Formation which perfected the Plaeozoic sequences in northern Tibet. According to these, we can realize tectonic control, palaeogeographrc pattern as well as palaeobiogeographic distribution of the Qinghai-Tibet Plateau in the early Paleozoic and also provided the materials to the cosmepolitical biostratigraphic contrast.